Is it possible to create a balloon rigid enough to contain a vacuum, but whose structure is not too heavy to not negate the bouyancy? How does feasibility change with scale?
I literally just 10 minutes ago saved a video on this exact topic to my Watch Later list o YouTube. Haven’t seen it yet, but here is the link. I’ll go watch it now and tell you what I find out 
So having watched the video – a vacuum filled (?) airship’s flotation mechanism would only be 20% more buoyant than a hydrogen airship’s, meaning you would need to build something with the same internal space as the Hindenburg except instead of a cloth baloon holding hydrogen you need to be capable of holding a rigid shape in the face of an atmosphere of pressure… while only increasing total weight by 20% over cloth and wood. So most likely impossible with current materials, but doable in the near future, yet only at immense cost.
Defintiely impossible with current materials. I’m not sure where you get the idea there’s some super rigid, yet very lightweight material in the offing. Do you know something we don’t?
Sorry for the triple post, but I did just think of another factor – how this would increase with scale. While a larger balloon would provide more buoyancy compared to surface area, I would think that it would also cause a similar increase in the stresses that are pushing the balloon to collapse. I am not sure if the stress will scale based on volume or surface area, though. If it scales based on surface area, then a truly humongous balloon might be doable?
I guess that near future is a relative term 
I was thinking of something like carbon nanotubes - I realize they aren’t particularly rigid but maybe they could be woven together in a specific way to increase rigidity. Even if the pattern has to be at a molecular level, it could hypothetically be possible.
I’m not saying anything like this will be mass produced in ten years, but I wouldn’t be surprise if we could make these sorts of materials in tiny quantities in a lab by then – which means that, theoretically at least, it would be possible to create these balloons “at incredible expense”.
I do realize we are probably more interested in super strong yet light materials rather than super rigid yet light, but who knows what we might discover.
You don’t need the skin to be rigid, just a air-proof unstretchable flexible membrane over a hard structure that holds it out.
here’s an article I found about super rigid, super strong, super light materials.Apparently they 3d print these shapes out of a blend of metals, ceramics, and other compounds.
If the force pushing down on the side of the container depends on the surface area rather than the total volume of the vacuum (which intuitively makes sense to me – but I could definitely be wrong) then you could build a skeleton out of this material, in one of those polyhedral shapes they show in the video. Then you can stretch some sort of airproof, tough, and light material around the shape - rigidity doesn’t matter here because the skeleton is providing that, it just has to be rigid enough not to bow inwards too far, and strong enough not to break.
I don’t know if these materials are light enough or not, but that seems to be a matter of degree. A more rigid yet lighter material could be developed later.
It can even be somewhat stretchable and somewhat air-permeable, you’d just need to account for less displaced air due to bowing and some device to keep pulling air out of the container faster than it can leak in.
A hard structure that is super rigid and lightweight. Same problem.
Right: a lightweight pump that can maintain a high vacuum in a huge space while consuming minimal power. Surely they’re sold (with free shipping) on eBay.
Carbon nano-tubes problem won’t solve this problem. half inch thick steel isn’t even strong enough for a railroad tank car.
Note that the failure mode being so drastic is another problem.
I recall about 15 years ago being shown a small shape memory form that was manufactured flat, but when heated expanded to a large volume (which, because the shape was hermetic, was filled with vacuum). It floated in air. I remember wondering if this was suitable for anything other than toys.
On a more practical note, here is a link to a thesis which concludes that it is just barely possible to build a practical LTA vehicle using vacuum in place of hydrogen or helium.
Well just make it bigger then. The more it is scaled up the more lift you get.
We’ve been over this before. There’s no material known which would work, and it’s scale-independent: A material which won’t work at one size won’t work at any other size, either. You’ll get more lift out of a larger structure, but it’s also harder for a dome to support pressure differences when it’s got a larger radius of curvature, so you need to make it thicker and heavier. And making a framework with a thin membrane stretched across it just makes it even worse: The framework then would need to be even stronger yet.
peccavi, I suspect you’re misremembering. Aerogels have a very low density, and the solid part of them is in fact less dense than air… but the bubbles between the solid parts are all full of air, for a total density that’s greater than air. And if you somehow remove that air, then it would float… except that it would collapse if you did that, because it’s not strong enough.
No- it was definitely not an aerogel. It was a shape memory alloy “sandwich” which was flat, but transformed into a volumetric shape. The floating version I saw was just a small thing, not useful for anything, so structural issues at that size were not apparent, at least in the short time I saw it. The person who showed it to me was a serial entrepreneur who had been an IBM scientist and then gone off to start a number of small companies (none of which got very big, but some of which are still around). He was excited that there was a manufacturing process that could produce the hermetically sealed flat shape memory alloy “sandwich”, but I couldn’t really see what it would actually be used for. Definitely not a practical method, but definitely not aerogel.
Edited to add: One of things that caused it to stick in my memory was that it was obviously (to me) inspired by the “lead balloon” approach to drastically reducing magnetic flux in cryogenic testing.
20%more lift is a huge deal. The Hindenberg didn’t use hydrogen over helium because of the unavailability of helium. It used it because of the lift. Most of the lift of a zepplin is used to lift the zepplin structure itself. The extra 8% or 9% lift translates to 10 or 20 times more cargo.
Could we get to work anywhere else?
For instance, Venus has slightly less gravity than Earth, but a much denser atmosphere. Ignoring the other (considerable) problems and concentrating purely on whether a vacuum balloon would work with current materials. Am I right in thinking that the dense atmosphere would make it easier?
The clouds of sulphuric acid probably aren’t going to help, mind.
It might be easier to build a vacuum balloon on Venus, but at the same time – why would we need to? Because of the denser atmosphere, things actually work out to where the lowest elevation you could reasonably float at without melting from heat or being crushed by pressure is right around where a large balloon filled with a mix of gasses mimicking our own atmosphere would settle. We could actually build our habitats to use our breathing air for buoyancy as well.